1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus that irradiates a subject placed in a static magnetic field with a radio-frequency magnetic field, detects a nuclear magnetic resonance signal emitted from the subject with a receiving coil, and reconstructs an image; and a receiving-path switching method.
2. Description of the Related Art
A Magnetic Resonance Imaging (MRI) apparatus is an apparatus that irradiates a subject placed in a static magnetic field with a Radio Frequency (RF) magnetic field, detects a Nuclear Magnetic Resonance (NMR) signal emitted from the subject owing to the irradiated radio-frequency field, and reconstructs an image.
Such MRI apparatus uses a receiving coil for receiving an NMR signal. There are various kinds of coils used for the receiving coil, such as a Whole Body (WB) coil, and a Phased Array (PA) coil that includes a plurality of surface coils (hereinafter, referred to as “coil element”), and an appropriate one is to be selected in accordance with a type of a scan.
Among imaging methods used by MRI apparatuses, there is a method of using a plurality of kinds of receiving coils. For example, according to parallel imaging that achieves a fast scan by receiving and processing signals in parallel by using a PA coil, a sensitivity prescan for creating a sensitivity map that indicates a sensitivity distribution of each coil element is performed prior to a main scan. A WB coil and a PA coil are used in the sensitivity prescan.
Specifically, in the sensitivity prescan, scanning is performed first by using the WB coil, and a WB-coil image is reconstructed based on an NMR signal received by the WB coil. Scanning is then performed by using the PA coil, and a PA-coil image is reconstructed from an NMR signal received by the PA coil. A sensitivity distribution of each coil element is then estimated by comparing the WB-coil image and the PA-coil image, and a sensitivity map is created (for example, see JP-A 2005-237702 (KOKAI), or JP-A 2005-237703 (KOKAI)).
In this way, conventionally, to perform a scan by using a plurality of kinds of receiving coils, scanning needs to be performed with each receiving coil, and a receiving path connecting a receiving coil and a receiving circuit (circuit that processes an NMR signal received by the receiving coil) needs to be switched scan by scan. For example, in a sensitivity prescan of parallel imaging, to begin with, the receiving path is set between the WB coil and the receiving circuit, and scanning for the first time is performed; and then after the receiving path is switched to a path between the PA coil and the receiving circuit, scanning for the second time is performed.
However, conventionally, because setting of the receiving path described above is performed scan by scan based on coil information selected by the operator, the subject sometimes may move while setting the receiving path in some cases. If the subject moves while setting the receiving path, there is a possibility of occurrence of misregistration (positional disagreement) among images each of which is reconstructed scan by scan, as a result, sometimes correct image cannot be obtained in some cases.
For example, according to a sensitivity scan of parallel imaging, when misregistration have occurred between a PA-coil image and a WB-coil image, a sensitivity distribution cannot be correctly estimated, resulting in a substantial influence on the accuracy of a sensitivity map.
To reduce such misregistration of images caused by a motion of the subject, it has only to switch the receiving path during a scan, instead of switching the receiving path scan by scan. Accordingly, it can reduce a time of a period having a possibility that the subject may move.